At present, there are three main treatment processes adopted for feed-water treatment of a boiler in power station at home or abroad, that is, the all volatile treatment (reduction) (AVT-R), the all volatile treatment (oxide) (AVT-O), and the oxygenated treatment (OT).
The AVT-R and the AVT-O can reduce the dissolution and deposition of the product of corrosion to some extent, but is significantly insufficient for inhibiting the flow accelerated corrosion (FAC) of a feed-water system. In particular, for some power sets adopting the AVT-R and the AVT-O, there are significantly excessive content of iron ions in the feed water, which directly leads to a series of problems such as high scaling rate, the deposition of iron scale on the blades of a steam turbine, etc. with regard to monotube type boilers, bringing about significant decrease in the efficiency of the power sets. Under this circumstance, most of the feed-water systems in monotube type boilers at abroad adopt relatively advanced OT process.
The OT process is initially developed by West Germany in 1970s. The guidelines for oxygenated treatment were officially established by the Welding & Repair Technology Center (WRTC) in the United States at the earlier 1990s. New guideline of oxygenated treatment was revised in China in 2011. In comparison with the AVT-O, the OT process can effectively inhibit FAC, since high purity oxygen is continuously supplied to the feed-water system in the OT process, which generates dense oxide films on the inner metallic wall of the feed-water system, and in turn effectively slows down the development of corrosion.
However, there are some disadvantages in the OT processes, that is, due to some concentration of dissolved oxygen maintained in the main steam, as required when performing the OT process (DL/T 805.1-2011 Guidelines of Oxygenated Feed-Water Treatment of Boilers requires a dissolved oxygen of ≧10 μg/L in the main steam), the peeling off of the oxide skin from the high-temperature heated surface (including the superheater, the reheater, the water cooling wall, etc.) made of some steels (for example, TP347) might occur, which may lead to the explosion of the heating pipes once being blocked by the oxide skin peeled off.